Micromechanical angular accelerometer with auxiliary linear accelerometer

ABSTRACT

A micromechanical accelerometer comprises a mass of monocrystalline silicon in which a substantially symmetrical plate attached to a silicon frame by flexible linkages is produced by selective etching. The plate has a plurality of apertures patterned and etched therethrough to speed further etching and freeing of the plate and flexible linkages, suspending them above a void etched beneath. The plate is capable of limited motion about an axis created by the flexible linkages. An accelerometer comprised of a substantially symmetrical, linkage supported plate configuration is implemented as an angular accelerometer paired with an auxiliary linear accelerometer, which is used to compensate for the linear sensitivity of the angular sensor, to achieve an instrument that is insensitive to linear acceleration and responds to angular acceleration.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 08/103,896, filed Aug. 6, 1993, now U.S. Pat. No. 5,473,945,which is a continuation-in-part of Ser. No. 07/904,211, filed Jun. 25,1992, entitled MONOLITHIC MICROMECHANICAL ACCELEROMETER, now abandonedwhich is a continuation-in-part of application Ser. No. 07/528,051,filed May 23, 1990, entitled MONOLITHIC MICROMECHANICAL ACCELEROMETER,Issued Jun. 30, 1992 as U.S. Pat. No. 5,126,812, which is acontinuation-in-part of commonly assigned U.S. application Ser. No.07/479,854, filed Feb. 14, 1990, entitled METHOD AND APPARATUS FORSEMICONDUCTOR CHIP TRANSDUCER issued Mar. 23, 1993 as U.S. Pat. No.5,195,371.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to accelerometers, in particular tomicromechanical accelerometers.

Semiconductor materials such as mono and polycrystalline silicon, andsemiconductor mass production fabrication techniques have been used toproduce micromechanical accelerometers, as in U.S. Pat. No. 4,483,194.More sophisticated devices employ more advanced technology to yieldmonolithic micromechanical accelerometers, as in U.S. Pat. Nos.4,805,456 and 4,851,080.

Advances in micromechanical transducer technology are described inApplicant's co-pending, commonly assigned U.S. patent application Ser.No. 07/372,653 entitled BRIDGE ELECTRODES; U.S. patent application Ser.No. 07/373,032 entitled RESTRAINTS FOR MICROMECHANICAL DEVICES; U.S.patent application Ser. No. 07/560,374, filed Jul. 30, 1990, entitledMOTION RESTRAINTS FOR MICROMECHANICAL DEVICES, Issued May 12, 1992 asU.S. Pat. No. 5,111,693; U.S. patent application Ser. No. 07/470,938,filed Jan. 26, 1990, entitled MICROMECHANICAL DEVICE WITH A TRIMMABLERESONANT FREQUENCY STRUCTURE AND METHOD OF TRIMMING SAME Issued Sep. 1,1992 as U.S. Pat. No. 5,144,184; and U.S. patent application Ser. No.07/493,327, filed Mar. 14, 1990 entitled SEMICONDUCTOR CHIP GYROSCOPICTRANSDUCER Issued May 14, 1991 as U.S. Pat. No. 5,016,172, all of whichare incorporated herein by reference.

The present invention is concerned with application of such advancesspecifically to accelerometers and further advancement of monolithicmicromechanical accelerometers.

SUMMARY OF THE INVENTION

A micromechanical accelerometer is disclosed comprising a mass ofmonocrystalline silicon in which a substantially symmetrical plateattached to a silicon frame by flexible linkages is produced byselective etching. The plate has a plurality of apertures patterned andetched therethrough to speed further etching and freeing of the plateand flexible linkages, suspending them above a void etched beneath.Additional apertures may be introduced to control damping in gaseousatmospheres. The plate has a weight disposed thereon near an end remotefrom the flexible linkages. The plate is capable of limited motion aboutan axis created by the flexible linkages. Stop means limit motion of theplate about the axis. Strain relief tension beams are provided torelieve stress induced by boron diffusion necessary to provide etchstopping and the tension beams are trimmable in a manner which permitstuning of the resonant frequency of the plate. Grooves or depressionsare provided in the flexible linkages to resist bending or bucklingwithout increasing torsional stiffness. The plate and flexible linkagesare electrically isolated from the silicon mass and frame by dielectricor P-N junction isolation. Integral P-N junction electrodes and surfacebridging electrodes may be used to provide top to bottom symmetry intorquing and sensing of the plate while maintaining isolation andmonolithic construction. Bias and readout circuitry used to sense andtorque the plate may be provided integrally with the plate and formedduring plate processing.

In alternative embodiments, a substantially symmetrical, linkagesupported plate configuration is implemented as an angular accelerometerpaired with an auxiliary linear accelerometer, which is used tocompensate for the linear sensitivity of the angular sensor, to achievean instrument that is insensitive to linear acceleration and responds toangular acceleration. The micromechanical devices have enhanced matchingof device characteristics as the angular and linear devices are bothfabricated from the same semiconductor substrate using micromechanicalfabrication techniques.

DESCRIPTION OF THE DRAWING

The invention will be more fully understood from the following detaileddescription of an illustrative embodiment taken in conjunction with theaccompanying drawing in which:

FIG. 1 is a top view of a micromechanical accelerometer according to theinvention;

FIG. 2 is a perspective view of a micromechanical accelerometeraccording to the invention;

FIG. 3A is a cross-section of flexible linkage grooves formed byanisotropic etching;

FIG. 3B is a cross-section of flexible linkage grooves formed by plasmaetching;

FIG. 4 is a cross-section view of the micromechanical accelerometer ofFIG. 2 taken along a line X--X and showing a toadstool motion restraint;

FIG. 4A is an expanded cross-section of the micromechanicalaccelerometer of FIG. 2 taken along a line Y--Y and showing aprotuberant motion restraint or dimple;

FIG. 4B is an expanded cross-section of an electrically isolated bridgeelectrode landing;

FIG. 4C is a top view of the bridge electrode landing of FIG. 4B havingsurface metalizations to effect a driven shield;

FIG. 5A is a diagrammatic representation of a sensor electrodeconfiguration of a micromechanical accelerometer according to theinvention;

FIG. 5B is a diagrammatic representation of a torquer electrodeconfiguration of a micromechanical accelerometer according to theinvention;

FIG. 6 is a circuit diagram for a torque system for use without opposingelectrodes;

FIG. 7 is a diagrammatic representation of accelerometer electronicshaving a feedback loop for rebalance;

FIG. 8A is a top view of an angular micromechanical accelerometerstructure;

FIG. 8B is a side section view, taken along line A--A of FIG. 8A; and

FIG. 9 is a diagrammatic view of an angular micromechanicalaccelerometer paired with an adjacent linear accelerometer;

FIG. 10 is a block diagrammatic representation of processing output fromthe linear and angular accelerometers;

FIG. 11 is a side view of a plate of a transducer having trimmableweights disposed at opposite ends thereof to effect a balanced proofmass;

FIGS. 12A, 12B and 12C are sectional representations taken at Y--Y ofFIG. 13, of initial silicon processing for fabricating transducers in analternative sandwiched fabrication process;

FIGS. 12D, 12E and 12F are sectional representations taken at X--X ofFIG. 13, of initial glass processing steps for fabricating transducersin an alternative sandwiched fabrication process;

FIGS. 12G and 12H are sectional representations taken at Y--Y of FIG.13, of device assembly for fabricating transducers in an alternativesandwiched fabrication process; and

FIG. 13 is a plan view of a micromechanical accelerometer fabricated inaccordance with the process depicted in FIGS. 12A-12H.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIGS. 1 and 2, a micromechanical accelerometer 20 comprisesa transducer element 22 which is suspended by a pair of flexiblelinkages 24 above a void 26 in a mass of silicon 28. The silicon mass 28is preferably an N-type epitaxialy grown layer on a base ofmonocrystalline N-type silicon. Selective doping and etching yieldsP-type transducer element 22 and flexible linkages 24 freely suspendedin a frame 30 above void 26.

Processes for selective doping and etching of silicon mass 28 aredescribed in the above-referenced applications.

Transducer element 22 includes a substantially symmetrical plate 32,torsionally supported by linkages 24, doped P-type and selectivelyetched free from silicon mass 28. A plurality of etch facilitating slots34 are formed through plate 32 to speed up and assure completeundercutting of plate 32 during formation of void 26 and release ofelement 22 from silicon mass 28. The pair of flexible linkages 24 havefirst ends 36 connected to silicon frame 30 and second ends 38 connectedto plate 32. The plate 32 is completely detached from silicon mass 28and free to rotate about an axis formed by the flexible linkages 24. Ifthe flexible linkages 24 are created by selectively P-type doping areasdefining the linkages 24 and plate 32, the resulting transducer element22 is effectively isolated from silicon mass 28 by the P-N junctionbetween P-type flexure and N-type mass 28 created by doping the flexureand plate areas.

Alternatively, dielectric isolation of transducer element 22 can beachieved by growing an oxide or silicon nitride layer or combinationthereof or the like over the N-type mass 28, as described in thereferenced applications, to form flexible linkages thereon. Dielectricisolation will provide much lower capacitance between the plate 32,flexible linkages 24 and the remainder of mass 28 permitting a devicehaving a significantly greater signal-to-noise ratio than a similardevice having P-N junction isolation of plate and flexures.

Plate 32 is substantially rectangular, being a typical size 300×600microns, and has a proof mass or weight 40 disposed near an end 41.Weight 40 is a two-piece asymmetric mass, being disposed at only one endof plate 32 remote from linkages 24. The weight 40 is provided byplating or otherwise depositing a surface metalization near end 41 ofplate 32. Preferably, weight 40 is located beyond electrodes, discussedhereinafter, to minimize the effects of temperature, which can cause themetallized end to bow and introduce inaccuracies into the device. Themetalization is done when other structures of the accelerometer aremetallized. Weight 40 provides sensitivity to acceleration forcesperpendicular to the plate 32.

Flexible linkages 24 attach plate 32 to silicon mass 28. The linkages 24are provided with depressions or grooves 42 which give the linkagesadded strength against buckling or bending while maintaining desiredtorsional flexibility. Depressions or grooves 42 may be etched intolinkages 24 anisotropically, in which case they have a "V"cross-section, as illustrated in FIG. 3A. Alternatively, the depressionsfor grooves 42 may be formed in linkages 42 by other processes known inthe art, such as plasma etching which provides a U-beam configuration asillustrated in FIG. 3B. After forming the depressions for grooves 42, ashallow selective P-type diffusion defining flexible linkages 24 alsodiffuses into the depressions and creates the "V" or "U" structure thatsurvives the final etch.

Each of the flexible linkages 24 has a tension relief beam 44, locatedproximate to its second end, formed by allowing etching of a slot 46through plate 32 during the final etch to free plate 32. The beams 44relieve tension created by boron doping of the linkages 24 and plate 32.Boron doping results in the etch resistance of structural elements,however, it also reduces lattice spacing of the doped P-type structureswith respect to the surrounding N-type silicon mass 28. The P-typestructure tries to shrink when undercut, but is restrained by thickermass 28, putting the flexible linkages 24 in tension. The latticespacing reduction coupled with a boron concentration gradient alsoresults in a slight bow in plate 32, which can be compensated for bymeans discussed hereinafter. Tension in the flexure or flexiblelinkages, is relieved by etching slots 46 through the plate 32. Asopposed to unrelieved linkages, the tension relief provided by beams 44results in linkages which are less stiff, enhancing the sensitivity ofthe device. The beams 44 created by slots 46 are trimmable in a mannerwhich permits tuning of the resonant frequency of plate 32. Trimming ofbeams 44, such as by laser ablation of plate 32 to elongate slot 46, isdescribed in the above-referenced application.

To facilitate sensing of acceleration-induced rotation and re-balancetorquing of plate 32, a plurality of electrodes are provided in themicromechanical accelerometer according to the invention. Symmetricaltorquing and sensing of plate 32 is preferable and is provided by havingpaired upper and lower torquing and sensing electrodes as shown in FIG.4. Four buried electrodes 48, 50, 52 and 54 are fabricated as dopedP-type regions on the surface of an underlying N-type mass 49 on whichthe N-type silicon mass 28 is grown. The P-type integral electrodes 48,50, 52 and 54 are positioned to lie beneath plate 32. Four bridgeelectrodes 56, 58, 60 and 62 span transducer element 22 asphotolithographically produced metalizations over resist layers aboveplate 32, as illustrated in FIG. 4A, in paired alignment with buriedelectrodes 48, 50, 52 and 54, respectively.

As illustrated in FIGS. 4, 5A and 5B, an inner pair of integralelectrodes 50, 52 and an inner pair of bridge electrodes 58, 60 are usedas sense electrodes, while an outer pair of integral electrodes 48, 54and an outer pair of bridge electrodes 56, 62 are used as torqueelectrodes.

Differential capacitance change, resulting from positional tilt of theplate 32 due to accelerations, is sensed by both pairs of senseelectrodes 50, 52, 58 and 60. Each pair of sense electrodes iselectrically driven 180° out-of-phase with respect to the other pairwith a 100 kHz frequency signal so that a net current out of the plateis obtained when the plate 32 tilts. The magnitude and phase of thecurrent from plate 32 is amplified and gives a measure of the angularposition of the plate 32. For this purpose electrodes 50 and 60 aredriven from a source 51 through inverting amplifier 53 while electrodes52 and 58 are driven through non-inverting amplifier 55. Plate 32 isconnected to an amplifier 63 having a feedback network 64. When theplate 32 is centered (i.e., not tilted by acceleration), capacitancesbetween the plate 32 and electrodes 50, 52, 58 and 60 are equal andthere is no net current at the summing junction of amplifier 63,connected to the plate 32 and thus no output voltage. When plate 32tilts, as a result of acceleration, the capacitances between the plate32 and electrodes 50, 52, 58 and 60 are unbalanced. A net current ispresent at the summing junction of amplifier 63 resulting in an ACvoltage output proportional to the change in differential capacitanceand thus tilt. Feedback network 64 allows the input of amplifier 63 tooperate at a virtual ground. Amplifier 63 and feedback network 64 arepreferably fabricated on the instrument chip. This configurationprovides minimum capacitive loading on the output node, reducesparasitic capacitance and noise pickup and, therefore, results in ahigher output signal-to-noise ratio. Access to the amplifier 63 is via ametalization to a reverse biased P-type flexible linkage 24.

Referring now to FIG. 5B, since the accelerometer is to be operatedclosed loop, positional information is fed to a control loop whichgenerates a voltage proportional to tilt angle. Another set ofelectrodes, outer pair of buried electrodes 48, 54 and outer bridgeelectrodes 56, 62 are used to apply a linearized analog of this voltage,differentially to the plate 32, electrostatically torquing the plate torestrain it against tilt.

A configuration of an accelerometer according to the invention may beimplemented using only buried or integral electrodes without opposingbridge electrodes. Such an embodiment is illustrated in an electricalcircuit model shown in FIG. 6.

Since the force on a capacitor is proportional to the square of theapplied voltage, it is desirable to linearize the net torque as afunction of the output voltage proportional to tilt angle beforetorquing the plate 32 by applying voltage to buried torque electrodes.

Linearization is done by using the following relationships:

    net torque: T=k[V1.sup.2 -V2.sup.2 ]

where V1 is the voltage applied to one torque electrode (or set ofelectrodes acting in the same direction), V2 is the voltage applied tothe other electrode, and k is a constant depending on the geometry, thedielectric constant, and the gap between the tilt plate and the torqueelectrode. Let:

    V1=B+E.sub.t

    V2=B-E.sub.t

where B is a fixed bias that holds the plate centered in the absence ofacceleration. Then combining these three, ##EQU1##

The torquing circuit shown in FIG. 6 implements the above algorithm,resulting in a linear relationship between the applied voltage E_(t) andnet torque T applied to the tilt plate. The bias voltage also serves tobias the P-N junction of the integral electrodes appropriately.

The circuit configuration of FIG. 6 is used in a feedback loop torebalance the tilt plate. A functional block diagram of the loop isshown in FIG. 7. An acceleration A_(in) impressed on the instrumentresults in a torque applied to the transducer which is a function of aplate pendulous mass and a pendulum length, represented as 80 in FIG. 7.Tilt angle of the plate is measured as a differential capacitance asdiscussed hereinbefore, by what is functionally an angle sensor 82. Theoutput E₀ of the angle sensor 82, is demodulated 84 with respect to the100 kHz excitation signal of the sensor. The DC output of thedemodulator is then passed through servo compensation 86 to anintegrator 88. The output of the integrator 88 is fed back to the torquenetwork 90 previously described, which serves to drive the tilt plateback so as to zero the output angle Θ. Any acceleration, A_(in), willapply a torque to the tilt plate equal to A_(in) *M*L, where M is thependulous mass and L the pendulum length. The loop then serves torebalance the tilt plate, resulting in a voltage on the integratoroutput E_(t). This voltage, required to hold the plate against theacceleration torque is the instrument output in volts, and isproportional to the input acceleration for a perfectly linear torquer.

It is advantageous to have integral and bridge electrodes opposed on thetop and bottom of plate 32. With either sense or torque electrodes ononly one side of the plate there is a net force between the plate 32 andthe substrate 28 which tends to pull the plate toward the substrate. Byusing two pairs of opposing electrodes, one pair on each side of thetilt plate, the electrodes can be connected as indicated in FIGS. 5A and5B to cancel out this force. Furthermore, because of the differentialnature of such a configuration the output of the sense electrodes andthe torque applied by torque electrodes, as functions of the angle ofplate 32, will be linearized.

It may be desirable to further enhance the integrity of signals derivedusing bridge electrodes, as illustrated in FIGS. 4-4C and effectingdriven shields thereunder. Each bridge electrode has opposite landingsor terminations represented by landings 106, 108 illustrated in FIGS.4A, 4B and 4C, disposed on the silicon frame. The electrode landings areformed over a metalization layer 110. An oxide layer 113 provides DCelectrical isolation but capacitance to the substrate is still large. Insome cases (i.e., where the bride electrodes are used at high impedancefor signal pickoff instead of excitation from a low impedance source)this capacitance adversely affects the signal.

The construction illustrated in FIGS. 4A, 4B and 4C provides a drivenshield to effectively neutralize this capacitance. As shown in FIG. 4C,electrical isolation region 111 is provided under the landing of thesignal pickoff bridge. In the discussion to follow it is understood thatboth regions 111, on both sides of the bridge, are treated the sameelectrically. Region 111 is DC-isolated by the surrounding P regions112, 114, which effect an isolating floor and wall, respectively. Region111 is driven at the same (or nearly the same) potential as electrodelanding 106 and the capacitance between them is thus electricallyneutralized, maximizing the signal-to-noise ratio at the point ofpickoff.

The extent of rotation of plate 32 about the axis formed by flexures 24may be limited by the implementation of mechanical stops, the functionand fabrication of which is discussed in detail in the referencedapplication entitled MOTION RESTRAINTS FOR MICROMECHANICAL DEVICES. Suchmechanical stops are illustrated in FIG. 4 which shows a side sectionalview of an accelerometer according to the invention including"toadstool"-shaped stops 70, 72.

Stops 70, 72, each have a post 74 and cap 76 disposed thereon. The posts74 are disposed within circular apertures 78 in plate 32 (best viewed inFIG. 1). Posts 74 must be fabricated of such dimensions such that plate32 moves freely with respect thereto and so as to account for any bowingof plate 32 due to crystal lattice spacing reduction resulting fromboron doping. The bottoms of posts 74 are each anchored to buried orcantilevered electrodes 79 which are maintained at the same potential asplate 32.

Alternatively, as shown in FIG. 4A, a micromechanical accelerometeraccording to the invention may include dimples 82 disposed at theextreme ends of plate 32 to effect mechanical motion restraint. Suchdimples 82 would be disposed protruding from beneath plate 32, above agrounded buried or cantilevered electrode 84. Like the toadstool stops,dimples must be fabricated of such a dimension so as to account forbowing of plate 32 while effecting motion restraint and should beapplied symmetrically relative to the plate.

Although asymmetry of the tilt plate is shown hereinbefore as resultingfrom a plated mass disposed thereon, it should be appreciated that suchasymmetry may be obtained by extending one end of the plate 32.

Although, in the embodiments discussed herein, the inner electrodesperformed sensing and other electrodes performed torquing, one ofordinary skill in the art may appreciate that these functions could beinterchanged so that outer electrodes sensed while inner electrodesprovided torque.

Although toadstool stops and dimples are discussed as motion restraintsin embodiments of an accelerometer according to the invention, othermotion restraining techniques may be implemented, such as cantileveredstops.

While a single micromechanical accelerometer is described andillustrated herein, it will be appreciated that a plurality of devicesas described can be used in conjunction in a redundant transducer andthat a plurality of transducer elements can be implemented in a singlemass of silicon working in conjunction to effect any of varioustransducer functions.

Referring now to FIGS. 8A and 8B, an angular accelerometer 800 isfabricated as a substantially rectangular plate 802 attached to a massof silicon 804, by a pair of flexible linkages 806. As with theembodiments described hereinbefore, the plate 802 and flexible linkages806 are formed by etching a void 808 in the mass of silicon 804 so thatthe plate 802 is freely suspended thereabove. The flexible linkageseffect an input axis coaxial therewith, about which the plate 802rotates in a limited manner. The flexible linkages or flexures 806 mayhave grooves disposed therein as discussed and in the illustrativeembodiment are fabricated in the same etching process that yields theplate 802. The flexures 806 are fabricated having sufficient stiffnessto substantially prevent the plate 802, which is configured to becapable of limited motion about the input axis created by the flexiblelinkages 806, from engaging the bottom of void 808 as the plate rotatesabout the linkages. If the flexures are made sufficiently stiff toprevent the plate from bottoming in the void in the presence of anexpected maximum angular acceleration, the tilt angle is a directmeasure of the angular acceleration.

The plate 802 is substantially symmetrical with respect to the flexiblelinkages 806. In the illustrative embodiment, a high degree of symmetryand uniformity is desirable to yield a balanced plate. A large proofmass is desirable as well. The large proof mass can be implemented witha large area structure, alternatively, the large proof mass can beimplemented by fabricating masses, such as metallizations, onto theplate as discussed hereinafter. Large area device fabricationtechniques, such as described in commonly owned U.S. Pat. No. 5,129,983entitled METHOD OF FABRICATION 0F LARGE AREA MICROMECHANICAL DEVICES,which is incorporated herein by reference, can be used to fabricate alarge area symmetrical plate having a large proof mass.

In the illustrative embodiment of FIGS. 8A and 8B, the plate 802comprises a proof mass that is a substantially balanced 10-20 micronthick and approximately 0.6 mm wide by 1.2 mm long silicon structure. Inthe presence of angular acceleration, the plate 802 will tend to tiltbecause of its inertia. The inertia of the large proof mass plate isgreat enough to provide the required angular acceleration sensitivity.

Sensing of the angular tilt is done capacitively by configuring theplate 802 as a first plate of a capacitor and using a plurality ofburied or bridge electrodes, described in the referenced applications,as the second plate of the capacitor. As illustrated in FIG. 8A, a firstpair of bridge electrodes 810, 812 is disposed proximate extreme ends ofthe plate 802. The first pair of bridge electrodes 810, 812 act as senseelectrodes, capacitively sensing displacement of the plate 802. Thecapacitance represented by the distance between each of the senseelectrodes 810, 812 and respective ends of the plate 802 provides adifferential readout. Circuitry (not shown) senses the differencebetween a first capacitor formed by a portion of the plate 802 and onesense electrode 810, and a second capacitor formed by another portion ofthe plate 802 and the other sense electrode 812. The difference betweenthe first and second capacitors yields a substantially linearized outputfrom the angular sensor 800.

In the embodiment illustrated in FIGS. 8A and 8B, a second pair ofbridge electrodes 814, 816 is used as electrostatic torque electrodes.The torque electrodes, each one disposed intermediate to one of thesense electrodes 810, 812 and the flexible linkages 806,electrostatically torque the plate 802 to rebalance. With the tilt plateelectrostatically torqued to rebalance, the tilt angle represents anerror signal and the voltage required to rebalance the tilt plate 802 isa measure of the angular acceleration.

A major difficulty in designing a practical angular accelerometer isthat any pendulous unbalance of the tilt plate, such as unbalance causedby asymmetry, results in a spurious output from the angularaccelerometer. The angular accelerometer 800 described hereinbefore,having matching pendulosity when completely balanced, is not responsiveto acceleration into the plane of the tilt plate 802. The angularaccelerometer 800 is sensitive to out-of-plane acceleration causingrotation of the plate 802 about its input axis. It is desirable in ahighly sensitive and accurate system to eliminate any error in theangular rate measured that is attributable to linear acceleration. Thus,referring now to FIG. 9, the angular accelerometer according to theinvention includes a linear accelerometer 900 adjacent to the angularaccelerometer on the same mass of silicon.

The second accelerometer 900 is a linear accelerometer fabricated in thesame mass of silicon and comprises a second tilt plate 902 supported bya second set of flexures 904 above a corresponding void. The secondaccelerometer 900 has a pendulous mass 906 fabricated thereon proximateto an end thereof, such as by deposition of a metallization. Thependulous mass 906 shifts the center of gravity of the second tilt plate902 and effects an asymmetrical transducer comprised of an unbalancedplate. The unbalanced plate 902 has an input axis that is normal to orout of the plane of the tilt plate 800 of the angular accelerometerdescribed hereinbefore making the device sensitive to inputaccelerations that are in the plane of the tilt plate 800 of the angularaccelerometer, providing a device sensitive to linear acceleration.

The linear acceleration applied to the unbalanced plate 902 resulting inrotation of the plate 902, is sensed by capacitive sensing using buriedand/or bridge electrodes as described hereinbefore and in the referencedapplications.

The substantially balanced angular accelerometer 800 and adjacentlydisposed linear accelerometer 900 are fabricated on the samesemiconductor chip which is approximately 2 mm by 4 mm. The adjacentdevices each occupy an area approximately 1 mm by 1.2 mm and are spacedapart approximately 1 mm. The output of the angular accelerometer 800 isreceived by circuitry, which also receives the output from the linearaccelerometer 900. The output of the angular accelerometer is correctedfor linear acceleration, to compensate for any unbalance and/or toremove the affects of linear acceleration, by adding or subtracting theproperly scaled linear accelerometer output from the angularaccelerometer output.

Referring now to FIG. 10, forces including angular acceleration (α) andlinear acceleration (a) are impressed on the transducers, which areresponsive as described hereinbefore. The angular accelerometer isresponsive, in accordance with its particular sensitivity, to the forcesimpressed thereon which results in an output voltage (E1) representativeof the applied acceleration. Similarly, the linear accelerometer, whichis subject to the same input as the angular accelerometer but hasdifferent sensitivity, effects generation of an output voltage (E2)representative of the acceleration applied thereto. The output voltagesE1 and E2 from the angular accelerometer and linear accelerometerrespectively, are defined as:

    E.sub.1 =G[P.sub.1 a+I.sub.1 α]                      (1)

    E.sub.2 =G.sub.2 [P.sub.2 a+I.sub.2 α]               (2)

wherein,

E₁ =output voltage associated with the angular accelerometer;

E₂ =output voltage associated with the linear accelerometer;

a=input linear acceleration;

α=input angular acceleration;

G=transfer function relating input acceleration and output voltage;

P=scale factor for linear acceleration;

I=scale factor for angular acceleration; ##EQU2## In practice, the P₁ I₂term in the denominator is usually small enough to neglect.

The angular accelerometer, as discussed hereinbefore, should be quiteuniform and symmetrical, while effecting a large proof mass. In order totrim the plate 802 for complete balance, a metal film can be depositedon the plate and selectively removed by laser ablation. Similarly, asillustrated in FIG. 11, the plate can be fabricated as a thin platehaving weights to increase sensitivity, such as tungsten metalizations,disposed proximate to opposite ends thereof. The tungsten weights can belaser trimmed to balance the proof mass.

Alternative methods can be used for fabricating an angular accelerometerwith linear compensation. As illustrated in FIGS. 12A-12G, anon-monolithic sandwiching process, as described in U.S. Pat. No.5,013,396 which is incorporated herein by reference, can be used toimplement the structure according to the invention. FIG. 13 is a planview of a device so implemented. A silicon substrate is first processed(FIGS. 12A-12C), a glass substrate is processed (FIGS. 12D-12F) and thesilicon and glass are bonded and etched (FIGS. 12G-12H).

As illustrated in FIG. 12A, a silicon substrate 920 is processed viaknown photolithographic and etching techniques to form a plurality ofmesas 922 thereon. The height of the mesas will eventually determine theelectrode to proof mass spacing. The tops of the mesas will be the areaof the silicon substrate that gets bonded to the glass. A borondiffusion is effected 924, as illustrated in FIG. 12B, to a depth ofapproximately 10 microns. The depth of this diffusion will determine thethickness of the proof mass and the flexures. The diffused substrate isetched via a reactive ion etch, to form what is ultimately the perimeterof the proof mass 926 and of the entire device, as illustrated in FIG.12C.

As illustrated in FIG. 12D, a glass substrate 928 is processed bypatterning resist 930 thereon. Recesses 932 are etched in the glass 928in portions unprotected by the resist, as illustrated in FIG. 12E. Metal934 is deposited through the resist into the recesses. Subsequently theresist is lifted off leaving the surface metallizations 934, asillustrated in FIG. 12F. The metallizations 934 are effectivelyelectrically isolated, as they are disposed in/on the glass.

Referring now to FIGS. 12G and 12H, the device is assembled by invertinga processed silicon proof mass 926, aligning it with the processed glassportion 928 and bonding the proof mass to the glass via an anodic bond.The assembled structure is then etched in EDP to remove all the undoped,unwanted silicon structure, leaving the desired accelerometer structure,as illustrated in FIG. 12H.

It should be appreciated that various other fabrication methodologiescan be practiced in implementing the angular accelerometer according tothe invention.

While a silicon plate/proof mass is described having laser trimmableweights or metallizations, it should be appreciated that the balancedplate of high mass can be alternatively fabricated by the deposition oftungsten over a silicon proof mass and then patterning the tungstenusing photolithography. Equivalently, nickel or gold can be selectivelyelectroplated through a photoresist spacer layer.

Although bridge and/or buried electrodes, as described in the referencedapplication, are discussed for sensing and torquing the plate of theaccelerometers herein, it should be appreciated that alternativetorquing and sensing techniques can be implemented, including conductivetraces disposed on the plate for magnetic force rebalancing of the proofmass, as described in co-pending commonly owned U.S patent applicationSer. No. 07/807,726 entitled ELECTROMAGNETIC REBALANCE MICROMECHANICALTRANSDUCER, which is incorporated herein by reference.

Although the invention has been shown and described with respect to anillustrative embodiment thereof, it should be understood by thoseskilled in the art that these and various other changes, omissions andadditions in the form and detail thereof may be made therein withoutdeparting from the spirit and scope of the invention as defined in thefollowing claims.

What is claimed is:
 1. A semiconductor micromechanical devicecomprising:at least one generally rigid structural element comprising afirst boron diffusion region of a substrate, said first boron diffusionregion having a first depth, and said at least one structural elementmechanically free from said substrate; at least one pair of flexureshaving first and second ends comprising a second boron diffusion regionof said substrate, said second boron diffusion region having a seconddepth smaller than said first depth, a first end of said flexuresmechanically free from said substrate and supporting said at least onestructural element, a second end of said flexures connected to saidsubstrate, said flexures controlling a drive or response motion of saidat least one structural element; and at least one electrode for sensingor exciting said motion of said at least one structural element, said atleast one electrode being located vertically proximate said at least onestructural element.
 2. The semiconductor micromechanical device recitedin claim 1 further comprising a proof mass mounted on said at least onestructural element.